Chem. Mater. 1993,5, 729-732
729
Crystal Structure and Magnetic Susceptibility of a Hydrocarbon Free Radical: Tris(3,5-di-tert-butylphenyl)methyl Bart Kahr,*9+Donna Van Engen,t and Prakash Gopalant Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393, and Department of Chemistry, Princeton University, Princeton, New Jersey 08544 Received February 9, 1993 The magnetic susceptibility and crystal structure of the toluene solvate of tris(3,bdi-tertbutylpheny1)methyl (1) were determined. Crystals of 1 G H 8 are monoclinic, space group C2/c: a = 11.476(4) A, b = 17.312(6) A, c = 22.216(7) A, /3 = 100.87(3)O, 2 = 4. The molecules have Czsite symmetry and approximate 0 3 symmetry in the lattice; the methyl carbon is planar in its coordination. Phenyl ring twists out of the mean molecular plane are comparable to other n e t a - or para-substituted triarylmethyls. The X-ray structure is in substantial agreement with the molecular structure calculated with the AM1 Hamiltonian. The magnetic susceptibility of the crystals is characteristic of a paramagnet; SQUID magnetometry shows no measurable exchange interaction above 2.6 K. This is consistent with a solvated lattice in which there is no appreciable n-overlap between adjacent radicals. A comparison with other known triarylmethyl crystal structures is reported. cia1
Introduction The bulk of alkyl substituents carried in a series of triarylmethyl biradicals was recently correlated with the sign of their exchange interactions in the solid state.l The groups presumably directed the stacking of the molecules pairwise, thus creating local interactions between siteswith like or opposite signs of spin density. Such correlations may be important for designing organic magnetic materials from stable paramagnetic molecules.2 A more complete understanding of the origins of the magnetic properties would necessitate detailed solid-statestructures. However, obtaining diffraction quality single crystals of highly reactive molecules can require good fortune. The title compound,tris(3,5-di-tert-butylphenyl)methyl (l),is unique among triarylmethyl radicals as it can be crystallized as a monomer or its associated ethane dimer, depending on the solvent from which it precipitates. Schreiner and coworkers grew bright red crystals of the benzene solvate of l.3They demonstrated the monomeric character of the molecules in the solid state by ESR. Stein et al. later observed that colorless crystals could be precipitated from cyclohexanesolutionsof 1.4 We reported the X-ray structure of the colorless crystals which featured the symmetrical ethane dimer, hexakis(3,5-di-tert-butylphenyl)ethane, associated with three cyclohexane mole c u l e ~ .Here, ~ we report and correlate the structure and magnetic susceptibility of the red crystals of 1. To our knowledge this represents the first crystal structure determination of a hydrocarbon in a doublet state. Purdue University. Princeton University. (1) Rajca, A.; Utamapanya, S.;Jiangtien, X. J.Am. Chem. SOC.1991, 113,9235. (2) McConnell, H. J.Chem. Phys. 1963,39,1910.Izuoka, A.;Murata, S.;Sugawara, T.; Iwamura, H. J. Am. Chem. SOC.1985,107,1786;1987, 109,2631. (3) Schreiner, K.;Berndt, A.; Baer, F. Mol. Phys. 1973,26,929. (4) Stein, M.; Winter, W.; Rieker, A. Angew. Chem., Int. Ed. Engl. 1978,17,692. (5) Kahr, B.; Van Engen, D.; Mislow, K. J. Am. Chem. SOC.1986,108, 8305. + f
acl181
21
@Ci191
I
161
Ci171 Cit31
ci151
C
ci211
C123I
Figure 1. View of tris(3,5-di-tert-butylphenyl)methyl(l)normal to the diad axis. Only the major tert-butyl positions are shown. Hydrogens have been deleted for clarity.
Diffraction and Susceptibility Studies Tris(3,5-di-tert-butylphenyl)methyl(l) crystallized from toluene as a 1:l solvate in the monoclinic system, space group C2/c: a = 11.476(4)A,b = 17.312(6)A, c = 22.216(7) A,P = 100.87(3)O,2 = 4. The radicals lie on the diad axes in the crystal and are necessarily planar at C(1). The included toluene and tert-butyl groups are disordered. These features, along with the extreme sensitivity of the crystals to oxidation and drying, presumably contributed to a high R value of 0.124. The atoms of the triphenylmethyl skeleton are, however, well-determined. A view of the major conformer of 1normal to the diad axis is shown in Figure 1.
0897-4756/93/2805-0729$04.00/00 1993 American Chemical Society
Kahr et al.
730 Chem. Mater., Vol. 5, No. 5, 1993
Table I. Fractional Coordinates and Isotropic Thermal Parameters (A2X lo3) for l.C,HR
E .
X
0
0.10
o.Ooo0
0.0000
0.0375(6) 0.0381(6)
v
0.05
0.0000
x
5 0
100
150
200
250
300
T (K) Figure 2. Plot of magnetic susceptibility for crystals of lCVHB, x vs T, with the field oriented along the crystallographic b axis. Inset shows xT vs T.
The magnetic susceptibility of several crystals was measured as a function of temperature between 2.6 and 350 K in an applied field of 0.1 T oriented along the crystallographic b axis. The plot of x vs T is shown in Figure 2. From a Curieweissfit, the molar Curie constant was extracted; a contribution of one spin per molecule results. The Curie temperature resulting from the fit is almost zero, suggesting paramagnetic behavior without any indication of magnetic ordering. The inset shows xT vs T. The function is essentially linear with a slight downturn at 10 K.
Discussion of Structure The solution structure of 1had been previously studied by ESR.3 Two important structural parameters may be obtained from magnetic resonance spectra: the out-ofplane twisting of the phenyl substituents (4) and the deviation from trigonal planarity (8). Carbon spin densities may be calculated from experimental 'H coupling constants using the McConnell equation6 and by the spinpolarization theory of Karplus and Fraenkel (KF).7 Falle et al. postulated that the twist angle of a triarylmethyl can be related to an excessive hyperconjugative or direct overlap contribution to the Cortho coupling constant which has a sin2 4 dependence.8 Schreiner et al.3 calculated an average phenyl ring twist in 1of 24' at room temperature using the spin densities obtained with KF proportionality constants, Q.7 Alternative Q values proposed by Strom, Underwood, and Jurkowitz (SUJIg gave a twist of 26°.3 The angles are in fair agreement with the crystal twist angles of 36.0' and 31.2' for the phenyl rings in 1with C2 and C1site symmetry, respectively. For comparison, the twist-angles for triphenylmethyl have been given as 4045O by gas phase electron diffraction.1° Deviations from trigonal planarity in solution may be evidenced by unusually large 13Cm&,hyl coupling constants. Differences between the calculated and experimental values may be attributed to direct Fermi contact resulting from finite s-character at Cmethyl. Fessenden" showed that a ~ ~ ~ was ~ ~proportional ~ , ( ~ ~to ~8 2 ~ . Schreiner , ~ ) et al.3 calculated a 3 O deviation from trigonal planarity in this (6) McConnell, H. M. J. Chem. Phys. 1956,24, 764. (7) Karplus, M.; Fraenkel, G. K. J. Chem. Phys. 1961,35, 1312. (8) Falle, H. R.; Luckhurst, G. R.; Horsfield, A.; Ballester, M. J.Chem. Phys. 1969,50, 258. (9) Strom, E. T.; Underwood, G. R.; Jurkowitz, D. Mol. Phys. 1972, 24, 901. (10) Andersen, P. Acta Chem. Scand. 1965, 19, 629. (11) Fessenden, R. W. J. Phys. Chem. 1967, 71, 74.
0.0785(6) 0.1226(7) 0.1820(7) -0.0245(7) -0.0305(6) -0.1093(6) -0.1392(6) -0.0862(6) -0.0053(6) 0.0203(6) -0.2279(4) -0.3350(8) -0.2700(8) -0.1657(8) 0.0547(2) 0.1198(8) -0.0393(7) 0.1470(8) -0.340(2) -0.262(4) -0.173(4) -0.350(2) -0.228(4) -0.209(4) -0.304(3) -0.155(3) -0.307(3) 0.1899(6) 0.019(1) 0.009(1) -0.054(1) 0.0463 0.1591 0.1714 0.0710 -0.0417 -0.070(2) -0.034 0.1201 0.1772 0.1107 -0.0128 -0.203(4)
Y
2
0.5076(5) 0.5918(5) 0.6341(4) 0.7139(4) 0.7520(5) 0.7603(4) 0.7090(5) 0.8148(4) 0.8076(4) 0.4661(3) 0.4982(4) 0.4602(4) 0.3887(4) 0.3544(4) 0.3942(4) 0.4953(2) 0.4405(5) 0.5750(4) 0.5005(6) 0.2778(2) 0.2899(7) 0.2165(4) 0.2490(6) 0.512(3) 0.449(3) 0.572(2) 0.464(3) 0.5832(7) 0.475(3) 0.430(1) 0.531(2) 0.557(2) 0.283(1) 0.2340(1) 0.233(1) 0.129(8)
0.25% 0.2500 0.2026 (3) 0.2019(3) 0.2500 0.1516(3) 0.1048(3) 0.1789(3) 0.1169(3) 0.1915(3) 0.1417(3) 0.0864(3) 0.0807(3) 0.1280(3) 0.1833(3) 0.0324(2) 0.0171(5) 0.0464(4) -0.0228(4) 0.1159(1) 0.0626(3) 0.0940(4) 0.1699(3) 0.059(2) -0.027(2) 0.018(2) 0.041(2) 0.039(2) -0.032(1) -0.000(2) -0.120(2) 0.052(2) 0.1281(5) 0.1692(6) 0.0570(5) 0.2325(7) 0.2058 0.2427 0.3062 0.3329 0.2960 0.2427(9) 0.1978 0.2132 0.2735 0.3184 0.3030 0.221(2)
0.0188
0.0190 0.0133 0.0075 0.0073 0.0232(1) 0.0067 0.0034 0.0167 0.0332 0.0364 0.019(3)
U
Equivalent isotropic (Idefined as one-third of the trace of the orthogonalized U,,tensor.
way using KF parameters to obtain the spin polarization contribution to the 13Cmethyl coupling constant. The ESR spectrum, however, is consistent with time-averaged D3 symmetry. The SUJ parameters indicate no deviation from trigonal planarity. The radical is crystallographically planar. The small angular deviations from 120° in the crystalstructure of 1(C(2)-C(l)-C(lO) = 119.3(4)', C(10)C(1)-C(10') = 121.4(7)O)are not revealing. The central Caryl-Cmethyl-Caryl angles were reported as 116-118' in the gas-phase structure of triphenylmethy1,'O but it was argued that these values were not significantly inconsistent with a planar geometry at C1because of thermal motion.l2 Five symmetrically substituted triarylmethy1 radicals were previouslystudied crystallographicallyincluding tris@-nitrophenyl)methyl(2),13 (perchlorotripheny1)methyl (3),14* and tris(2,6-dimethoxyphenyl)methyl (4).15 Tris(l,3,5-trichlorophenyl)methyland tris(2,3,5,6-tetra~~
~~
(12) Bastiansen, 0.;Skancke, P. N. Adu. Chem. Phys. 1961, 3, 323. (13) Andersen, P.; Klewe, B. Acta Chem. Scand. 1967,21,2599; 1962, 16, 1817.
Magnetic Susceptibility of a Hydrocarbon Free Radical
Chem. Mater., Vol. 5, No. 5, 1993 731
Figure 3. Stereoview of the unit cell of 1C7HB along the c axis. Only one orientation for the tolyl rings is shown as is only the major orientation of the tert-butyl groups. Hydrogen atoms have been deleted for clarity. Table 11. Selected Structural Parameters for Tris(3,5-di-tert-butylphenyl)methyl(l), Tris(pnitropheny1)methyl (2), (Perchlorotripheny1)methyl (3). and Trid2.6-dimethox~~hen~l)methvl (4) ~~~~~
~~
~
~~
~~
~~
1
X-ray space group symmetry Cmethyl-Caryl Caryl-cmethylCaryl(deg)a
ESR AM1
2
3
4
C21c Pbcn P1 P2In Cz D3b 0 3 Cz Ci Cz 1.446 1.48(4) 1.47(1)c 1.419(6) 1.46(1) 1.469(7) 1.46(2) 1.481(4) 119.3(4) 117 120 118(1) 111.7(4) 121.4(7) 124(1) 124.2(4) 31.2(5) 24-26 32 30 46.3 12.2(2) 36.0(8) 40 53.4 61.0(1) 53.8
out-of-plane aryl ring twist (deg)a a Multiple values represent symmetry independent parameters. Time-averaged symmetry in solution. Average value.
*
chloropheny1)methyl were also reported, but they are substantially similar to 4.14b In addition, the structures of Thiele's and Chichibabin's biradicals were recently reported.16 All of these substances have essentially planar coordination a t the central methyl carbon; 1 , 2 (Pbcn, 2 = 4), and 4 (P2/n, 2 = 2) are crystallographically planar. The phenyl twist angles of 2 compare favorably with 1. The special ring is twisted by 40° and the general rings are twisted by 30°. The three independent twist angles of 3 (46.3,53.4, and 53.8O) are considerably larger because of the steric demand of the o-chlorine atoms. A large twist angle of 64" was calculated from the 13Cmethylcoupling constant in the ESR spectrum. The Cmethyl-Caryl bond lengths for 1 (1.458(12), 1.469(7) A) do not reflect differences in conjugation as in 4 (1.419(6), 1.481(4) A) which features a marked departure from D3 sy"etry.15 The ground-state structure of 1 was optimized using the AM1 Hamiltonian.'' The calculated structure is in substantial agreement with the crystallographic structure. A comparison of the X-ray, ESR,and AM1 geometries of (14) (a) Veciana, J.; Carilla, J.; Miravitlles, C.; Molins, E. J. Chem.
Soc., Chem. Commun. 1987, 812; J . Inclus. Phenom. 1987, 5 , 241.
(b) Armet, 0.; Veciana, J.; Rovira, C.; Riera, J.; CastaAer, J.; Molins, E.; Ruis, J.; Miravitlles, C.; Olivella, S.; Brichfeus, J. J.Phys. Chem. 1987, 91, 5608. (15) Kahr, B.; Jackson, J.; Ward, D.; Jang, S.-H.; Blount, J. Acta Crystallogr., Sect E. 1992, Ea,324. (16) Montgomery, L. K.; Huffman, J. C.; Jurczak, E . A.; Grendze, M. P.J. Am. Chem. SOC.1986,108,6004. (17) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J . Am. Chem. SOC.1985, 107, 3902.
1as well as a summary of salient structural features of the triarylmethyl radicals, 2,3, and 4, are presented in Table 11. A stereoview of the lattice of lC7H8 is shown in Figure 3 along the c axis. The toluene molecules lie approximately in the ac plane forcing large distances between the mean molecular planes of theradicals (11.476(4)A). The shortest intermolecular C(l)-C(l) distance is 10.39 A, confirming the monomeric nature of the radicals. Similarly, the shortest intermolecular C(l)-Cpwa distance is 7.25 A between C(1) and C(5), thus excluding the possibility of nascent a,p association in the crystal.18 The absence of cooperative magnetic phenomena is in agreement with the remote separation between spin centers imparted by the included solvent molecules.
Experimental Section Crystallography. Crystals of tris(3,5-di-tert-butylphenyl)methyl (1) were prepared as described previ~usly.~ The orangered crystals had well-developed {OlO)faces. They were extremely air sensitive and effloresced rapidly when outside their mother liquor. Furthermore, evaporation of the saturated liquor from the surfaces of the wet crystals precipitated hairlike coatings of microcrystals. These obstacles to mounting a crystal suitable for diffraction were overcome in the following way. First, a single crystal was extracted from the bottom of the liquor with a spatula inside an argon-filled polyethylene glovebag. The crystal was transferred to a small container filled with dry toluene at -78 ' C for purposes of washing and to prevent drying. It was then inserted, rapidly, in a 0.3-mm Lindemann glass capillary containing a small volume a toluene. The capillary was removed from the glovebag after sealing the open end with clay. Finally, flame sealing secured the crystal in a vapor-rich environment. The capillary was transferred immediately to the goniometer placed under the cold nitrogen stream. Cell constants and their esd's were determined by a leastsquares fit of 21 diffractometer-measured reflections with 20' 5 26 5 25'. The material belongs to the monoclinic crystal system, space group C2/c, with a = 11.476(4) A, b = 17.312(6) A, c = 22.216(7) A, and = 100.87(3)'. A density of 1.03 g cm-3 was calculated for 2 = 4, M = 672.1 g, and V = 4334(2) A3, All intensity measurements were made at low temperature (175 A 3 K) using graphite-monochromated Mo K a radiation (A = 0.710 69 A) and an w-scan technique with a variable scan rate. Background counts were taken at each extreme of the scan range. All data (3228) having h, k 1 0 with 3' 5 28 I 45' were measured (18) Lankamp, H.; Nauta, W. Th.; MacLean, C. Tetrahedron Lett. 1968, 249.
732
Chem. Mater., Vol. 5 , No. 5, 1993
in this manner. Crystal decomposition was monitored throughout data collection by periodically remeasuring standard reflections; no significant variations in intensity were observed. The intensities were reduced by applying Lorentz and polarization corrections. Systematically absent reflections were eliminated and equivalent reflections were averaged to give 2837 unique data of which 1811 with pol> 30(F0) were considered to be observed. The structure was solved by direct methods (SHELX86) in the space group C2/c. Alternating cycles of least-squares refinement and difference Fourier map calculations led to the identification of carbon atoms and phenyl hydrogen atom positions. Following refinement with isotropic temperature factors, a difference Fourier map displayed a number of significant peaks corresponding to disorder of the C(16) and C(20) tertbutyl groups and further disorder of the toluene molecule. Four unique conformations were identified for (3171, C(18),and C(19) and twoforC(21),C(22),andC(23).Theoccupancyfactorswere initially allowed to vary and were then fixed at the refined values of 0.64,0.12,0.12,0.12 for C(17), C(18), C(19) and 0.67,0.33 for C(21), C(22), C(23). The occupancy factors for the toluene positions were fixed at 0.25. The toluene phenyl groups were refined as rigid rings (C-C 1.395 A, C-C-C 120O)with an isotropic temperature factor. Hydrogens were subsequently included in the refinement a t idealized positions (C-H 0.96A, C-C-H 109.5O) and fixed. Refinement converged (mean shift/error I 0.1) a t R = 0.124, R, = 0.137. A final difference Fourier map displayed four large residual peaks (0.45-0.99 e k3), all in the vicinity of the disordered tert-butyl groups. Magnetic Susceptibility. Magnetic measurements were carried out using a SQUID magnetometer (Quantum Design). Single crystals were sealed in a thin film of epoxy in a glovebag purged
Kahr et al. with Nz. Several crystals (-0.1 mm3) were stacked upon oneanother such that their b axes were oriented in a copardel fashion. They were then inserted in a quartz ESR tube that was subsequently sealed with wax. The tube was mounted in a straw that served as the sample holder. The magnetic field was oriented along the common b direction. The temperature dependence of the magnetization was measured between 2.6 and 350 K in afield of 0.1 T. At each temperature, the data point was collected as an average of three scans, after thermal equilibrium had been attained. For the data collection, a scan length of 4 cm and 32 points/scan were used. The data were corrected for the core diamagnetic contribution and that resulting from the sample holder. A M-H curve was recorded at 2.6 K in fields between -2 and 2 T in increments of 0.1 T.
Acknowledgment. We thank the National Science Foundation for a grant (CHE-9114265) awarded to B.K. and James E. Jackson (Michigan State University). We are indebted to Kurt Mislow for the generous financial support of the work carried out at Princeton University (CHE-8510067). We appreciate Stanley Dostal for his assistance and thank Professor Michael McElfresh for the generous use of the SQUID. SupplementaryMaterial Available: Bond lengths, valence angles, with standard deviations, anisotropic thermal parameters for carbon, and positional parameters for hydrogen for 1 (4 pages); table of observed and calculated structure factors (6 pages). Ordering information is given on any current masthead page.
